The present invention relates to a hydraulic master cylinder for a motor vehicle braking circuit.
The master cylinder of a motor vehicle braking circuit is supplied with hydraulic fluid (brake fluid) from a reservoir and is controlled from the brake pedal to supply hydraulic fluid under pressure to the means for applying the brakes on the wheels of the vehicle.
Typically, the master cylinder comprises a piston (the primary piston in the case of a tandem master cylinder) which can be moved axially in a working chamber (primary chamber) by boost means operated by the brake pedal, and comprises an axial passage for communication between the working chamber and a supply chamber connected to the aforesaid reservoir. A valve shutter urged by a spring is borne by the piston to close the end of this passage which opens into the working chamber once the piston has covered a dead travel starting from its position of rest.
The closing of this passage by the valve shutter makes it possible, with the piston travel continuing, to cause the pressure in the working chamber to rise so as to actuate the means that apply the brakes. After a braking action, the master cylinder piston is returned to its position of rest and the valve shutter re-opens the communication between the working chamber and the supply chamber connected to the reservoir.
The opening of the valve shutter may also be brought about by a depression in the working chamber, this depression resulting from the operation of a circuit of the ESP (Electronic Stability Program) type which selectively operates the brakes, this circuit comprising at least one pump supplied with hydraulic fluid by the outlet from the master cylinder.
A tandem master cylinder comprises a secondary chamber for supplying pressurized fluid to a secondary braking circuit and a secondary piston which can be moved in the secondary chamber in response to the movement of the primary piston in the primary chamber or aforesaid working chamber. Like the primary piston, the secondary piston has a passage for communication between the secondary chamber and a fluid supply chamber, and a valve shutter urged by a spring is borne by the secondary piston to close off this passage for braking, once the secondary piston has covered a dead travel starting from its position of rest.
Attempts have already been made at reducing and controlling the dead travels of master cylinder pistons, because these dead travels delay the onset of braking and vary with manufacturing and assembly tolerances. However, to do this, measurement and adjustments have to be made for each individual master cylinder, and this is relatively lengthy and expensive.
It is a particular object of the present invention to avoid these disadvantages of the prior art.
The invention for this purpose proposes a hydraulic master cylinder for a motor vehicle braking circuit, comprising at least one piston that can move axially in a working chamber and is formed with an axial passage for communication between said working chamber and a brake fluid supply chamber, and a valve shutter urged by a spring to close said passage when the piston has covered a dead travel in the working chamber starting from a position of rest, characterized in that it comprises means of concealing the aforesaid dead travel making it possible to reduce the flow of fluid through the aforesaid axial passage so as to increase the pressure in the working chamber during the dead travel of the piston, these means being borne by the valve shutter.
The invention therefore makes it possible to conceal the dead travel of the piston by causing the increase in pressure in the working chamber to begin as soon as the piston starts to move, before the end of its dead travel, this having the advantage of not delaying the onset of braking without in any way interfering with the other stages of operation of the master cylinder.
In a preferred embodiment of the invention, the means of concealing the dead travel comprise an additional valve shutter urged by a spring to close an end duct in the piston in which the aforesaid passage opens, this additional valve shutter comprising means allowing a reduced flow of fluid in said passage and said duct when the additional valve shutter is in its position for closing the duct.
Advantageously, these means allowing reduced flow of fluid are formed by at least one orifice passing through the additional valve shutter.
Such a valve shutter preferably consists of a washer which is mounted so that it can move axially on an axial rod secured to the first valve shutter and which is made of a rigid material such as metal or a plastic resistant to brake fluid.
The means of concealing the dead travel of the master cylinder piston according to the invention are therefore of simple structure, easy to install and inexpensive. Fitting them requires no measurements to be taken beforehand on the master cylinder and their operation is not dependent on manufacturing and assembly tolerances.
In the case of a tandem master cylinder comprising primary and secondary pistons which are able to move respectively in primary and secondary chambers to supply primary and secondary braking circuits, with valve shutters borne by the primary and secondary pistons and urged by springs to close passages in these pistons which connect the primary and secondary chambers respectively to brake fluid supply chambers, means for concealing the dead travels of the primary and secondary pistons are provided to reduce the flows of fluid through the passages in the primary and secondary pistons and to increase the pressure in the primary and secondary chambers during these dead travels, these means being borne by the pistons and consisting of additional valve shutters of the same type as the one described hereinabove.
Advantageously, the valve shutters of the primary and secondary pistons of the master cylinder are identical and, likewise, the additional valve shutters of the concealment means are identical.